Various direct voltage converters, which are sometimes also called direct voltage transformers, are known in order to convert direct voltages in other direct voltage ranges. For certain applications, such as for battery charging devices or battery test devices, powerful direct voltage converters are required, which may also be operated in a bidirectional way. A possible implementation of a bidirectional direct voltage converter is represented by a synchronous converter, which represents a step-down-converter, i.e. which converts input voltages into output voltages, which are smaller or at most equal to the input voltages, or vice-versa. To this end, a half bridge is used, wherein the two power switches of the half bridge are actuated by a pulse width modulation control unit (PWM control unit), in a way that the power switches switch within a switching period, which is defined by the predetermined switching frequency, alternately actuated with inverted switching pulses. Thus, basically, one power switch of a half bridge is always conductive, while the other one is disabled. The value of the output voltage of the synchronous converter is set by targeting of the duty cycle of the PWM control unit. The duty cycle indicates the ratio of the pulse duration of the upper power switch of a half bridge to the switching period. These synchronous converters may also be implemented as multi-phase converters. In this embodiment, as known, a plurality of half bridges, respectively being provided with two power switches, are connected in parallel via the output inductors of the half bridges and are actuated, in sequence, by the PWM control unit. Thus, the power switches are usually actuated with a cycle offset, which is equal to a fraction of the switching period, primarily for reducing output current ripples. The output current in multi-phase synchronous converters is determined with a fixed sampling rate. In order to ensure that the sampling points respectively fall on the current average values, the cycling of the power switches occurs with an offset corresponding to the switching period duration divided by the number of half bridges. With multi-phase actuation, larger output currents may be generated with smaller current ripples, or the switching frequency of the sum output current of such a multi-phase converter may be reduced, thus allowing a simpler filter sizing. Last but not least, in a multi-phase converter, smaller capacitors may be used for the same power range.
However, it is known that the upper and lower power switch of a half bridge cannot be simultaneously switched to the conducting state, since the voltage input of the synchronous converter would otherwise be short-circuited. For this reason, it is not possible to switch both power switches of a half bridge simultaneously or immediately in sequence, since otherwise, for example due to switching delays in the nano/microseconds range, both switches would be at risk of being both, in a conductive state for a short time, thus short-circuiting the input. Remedy is provided in the form of dead times between the switching of the upper and lower power switch of a half bridge. Thus, it may be ensured that one switch is switched off before the other is switched on. By introducing dead times, a reduction of obtainable duty cycle is however always caused, since the obtainable minimum and maximum duty cycle and thus the minimum and maximum output voltage are thus limited. The covered voltage range of the synchronous converter is thus reduced in case of actuation with inverting PWM signals and it is impossible to output particularly small or large voltages. The actuation of the half bridge with inverting PWM signals is mainly for allowing the current within a pulse period to continuously switch between positive and negative values. Since in normal operation both power switches are always alternately switching, a high or low output voltage cannot be set or obtained in a precise way, since the voltage is influenced by the required pulses of the other power switch and the consequently generated dead times. Thus, no theoretical maximum or minimum possible voltage may be output on the output side, without causing a certain error. PWM controls are already known, which at least allow a duty cycle of 100 percent, although a gap is present at values slightly below 100 percent.